The present invention is with respect to an apparatus for running off liquids from sealed vessels, and more specially to such an apparatus for use with sterile sealed vessels, having a liquid duct for running off liquid from the vessel, an air duct for letting air into the vessel from the outside for pressure compensation within the vessel, and an air filter joined up with the air duct.
Such apparatus is used in the medical field where it is known as universal infusion apparatus. It is so designed that liquid may be run off from the vessel under all likely working conditions. When using such apparatus the place of the volume of liquid run off from the vessel is taken by air from the outside. For ruling out the chance of microbes present in the outside air from making their way into the inside of the vessel in the air which is let into the vessel to take the place of the run-off liquid, there is an air filter through which such necessary air has to go before making its way into the infusion apparatus and then on through the air duct into the inside of the vessel. This air filter furthermore has to have the function of a liquid seal, because for example in the case of pressure infusion, when there is a gage pressure in the vessel, liquid will make its way as far as the air filter, that for this reason is made of hydrophobic material stopping, because of its nature, any loss of liquid out of the infusion apparatus.
Even although for pressure-less infusion vessels are used in the case of which the place of the run-off liquid is not taken by the vessel collapsing but by taking in air from the outside, a certain degree of gage pressure is still necessary within the vessel for forming the first air bubble on the air filter and for this reason for increasing the size of the liquid surface, before the vessel is able to take up air, that is to say before the filter is "primed" or its operation started. On the other hand collapse of many of the thin-walled infusion vessels is likely even at very low degrees of vacuum, so that the intake of air has to be got going with a vacuum level, that is less than this. Seeing that the degree of vacuum is dependent on the material and pore size of the air filter, this part of the design has been taken care of by using filters designed for this purpose.
There is a shortcoming here however inasfar as such known air filters are only water-tight up to a pressure of less than 0.5 bar. This being so, universal infusion apparatus is generally designed with air filters that are liquid-tight even at gage pressures likely on pressure infusion. However, when used for pressure-free infusion, such air filters have a higher gage pressure for forming the first air bubble. On the other hand with most of the very thin-walled vessels used for pressure-free infusion collapse is likely even at a degree of vacuum that is markedly less than the vacuum needed for forming the first air bubble. Seeing that universal infusion apparatus is used not only for pressure-less infusion but furthermore for pressure infusion, there is the shortcoming with known forms of such apparatus that they are not fluid-tight enough for pressure infusion or they do not let air into thin-walled vessels early enough for stopping them collapsing.
Even although attempts have been made at taking care of this shortcoming by fitting a check valve with a ball or a rubber sealing part for the purpose of stopping infusion liquid from running back through the air filter, such a valve has the undesired property of needing a certain pressure before opening and of being dependent on the position or angle of the apparatus for its operation. Furthermore sticking of the valve in position is likely when it is used for pressure infusions or if the apparatus is not used for some time.